A vehicle is a mobile machine that transports people or cargo. Typical vehicles include land vehicles, watercraft, aircraft, spacecraft, and the like. The land vehicles include wheeled vehicles, tracked vehicles, railed vehicles, skied vehicles and the like. A large portion of vehicles are equipped with one or more electrical loads. For example, many commercial and industrial trucks are equipped with a liftgate (or tailgate), a refrigeration unit, a trash compactor, a truck bed lifter, a snowplow, a lighting system, and/or the like. Electric power for operating such electrical loads is typically provided by the vehicle's electrical system. By way of an example, when a vehicle's ignition power is on, an alternator supplies electrical power to the vehicle while keeping the vehicle battery charged sufficiently. When the engine is not running, the electrical load draws electrical power solely from the vehicle battery. While being on a mission, the vehicle batteries can be discharged to the point where they cannot operate the liftgate anymore, and consequently will be incapable of powering the electrical load and starting the engine. Accordingly, there is a need for a power system that provides sufficient power to electrical loads in a more stable and predictable manner.